Hey Rok, moving past your need / desire for a surveillance system..............
You might consider asking the friend that's currently serving on the police force to put you in touch with a Detective assigned to drug enforcement in your city or county. It sounds like you might live in a metro area so the regular patrol officers on duty are usually stretched pretty thin and don't have the bandwidth to do any effective investigating or crime prevention. As you have noticed, they are usually late to the party and present only after a crime has been committed.
I can't help but think that you and your family may be living in or near a dangerous situation. Your story shouts illegal activity and to me illicit drug sales or use are the primary suspects here.
If you can't get any help from the local police department, I'd recommend you contact the county sheriff's office and run it through the chain of command there. In the event none of the local or county law enforcement agencies will get the ball rolling, your next step would be to contact the DOJ and DEA. Depending on your locale, there may be field office nearby that can investigate the situation.
It's worrisome to me that your neighbor seems aware that you are monitoring their activities. If this guy is indeed selling drugs, especially harder stuff like meth, heroin, coke, opioids etc. there is a real danger to you and your family's safety. Either from him, or those "customers" frequenting his "business".
My main point........ I don't think that surveillance monitoring is the only solution you need here.